The partial substitution of coal with biomass in the existing power plant fleet can reduce the carbon dioxide pollution in a cost effective way at high efficiencies. The goal of this study was to identify suitable co-firing ratios during co-combustion of straw and three different coal types (0, 10, 25, 40, 60, 100% straw on energy basis in fuel blend) minimizing the corrosion risk. An online corrosion monitoring systems based on the linear polarisation resistance method has been used for measurements in two pulverised fuel co-firing systems: An externally heated entrained flow reactor at the Technical University of Munich and a 300 kWth drop down fired reactor at University of Stuttgart. The experiments at the entrained flow reactor have been conducted for 8 h per blend at 1200 °C with an excess oxygen of 3-5 vol-%. Online corrosion measurements were performed at a flue gas temperature of about 700 °C – 900 °C representing the superheater region of a power plant. Material temperature was set to 530 °C utilizing 10CrMo9-10 as the examined alloy. The pilot scale tests were performed for 0, 25, 40, 60% e.b. straw content. Each blend was combusted for 24 hours. At lab scale tests a constant sensor signal was measured for 0, 10, 25, 40% e.b. straw content which increased steeply after changing the ratio to 60%, indicating a corrosive atmosphere. This phenomenon was found for all three different coals, however the overall corrosion signal was lower for coal from South Africa and El Cerrejon. Nevertheless, all series of measurements showed an increasing signal for a higher share of straw in the fired fuel blend. These tendencies were also confirmed at the pilot scale test with El Cerrejon coal. Deposition and fuel analysis show correlations between the Cl-content in fly ash, Al2O3 content in the fuel and the corrosion signal.